PROJECTIONS OF POINTS A point may be situated

PROJECTIONS OF POINTS

A point may be situated, in space, in any one of the four quadrants formed by the two principal planes of projection or may lie in any one or both of them. Its projections are obtained by extending projectors perpendicular to the planes. One of the planes is then rotated so that the first and third quadrants are opened out. The projections are shown on a flat surface in their respective positions either above or below or in xy.

Four Cases: 1. The point is situated in the first quadrant. 2. The point is situated in the second quadrant. 3. The point is situated in the third quadrant. 4. The point is situated in the fourth quadrant.

A POINT IS SITUATED IN FIRST QUADRANT h a’ y o d x a

The pictorial view shows a point A situated above the H. P and in front of the V. P. , i. e. in the first quadrant. a' is its front view and a the top view. After rotation of the plane, these projections will be seen as shown in fig. (ii). The front view a' is above xy and the top view a below it. The line joining a' and a (which also is called a projector), intersects xy at right angles at a point o. It is quite evident from the pictorial view that a'o = Aa, i. e. the distance of the front view from xy = the distance of A from the H. P viz. h. Similarly, ao = Aa', i. e. the distance of the top view from xy = the distance of A from the V. P viz. d.

A POINT IS SITUATED IN SECOND QUADRANT b x o d h b’ y

• A point B in (fig. ) is above the H. P and behind the V. P. , i. e. in the second quadrant. b' is the front view and b the top view. • When the planes are rotated, both the views are seen above xy. Note that b'o = Bb and bo = Bb'.

A POINT IS SITUATED IN THIRD QUADRANT d c y o h x c’

• A point C in (fig. ) is below the H. P and behind the V. P. i. e. in the third quadrant. Its front view c' is below xy and the top view c above xy. • Also co = Cc and • co = Cc'.

A POINT IS SITUATED IN FOURTH QUADRANT o y h d x e’ e

• A point E in (fig. ) is below the H. P. and in front of the V. P. , i. e. in the fourth quadrant. Both its projections are below xy, and e'o = Ee and eo = Ee'.

Prob. 1. Draw the projections of the following points on the same ground line, keeping the projectors 25 mm apart. A, in the H. P. and 20 mm behind the V. P. B, 40 mm above the H. P. and 25 mm in front of the V. P C, in the V. P. and 40 mm above the H. P. D, 25 mm below the H. P. and 25 mm behind the V. P. E, 15 mm above the H. P. and 50 mm behind the V. P. F, 40 mm below the H. P. and 25 mm in front of the V. P. G, in both the H. P. and the V. P.

Prob. 2. A point P is 15 mm above the H. P. and 20 mm in front of the V. P. Another point Q is 25 mm behind the V. P. and 40 mm below the H. P. Draw projections of P and Q keeping the distance between their projectors equal to 90 mm. Draw the straight lines joining (i) their top views and (ii) their front views.

Prob. 3. The two points A and B are in the H. P. The point A is 30 mm in front of the V. P. , while B is behind the V. P. The distance between their projectors is 75 mm and the line joining their top views makes an angle of 450 with xy. Find the distance of the point B from the V. P.

Prob. 4. A point P is 20 mm below H. P. and lies in the third quadrant. Its shortest distance from xy is 40 mm. Draw its projections.

Prob. 5. A point A is situated in the first quadrant. Its shortest distance from the intersection point of H. P. , V. P. and auxiliary plane is 60 mm and it is equidistant from the principal planes. Draw the projections of the point and determine its distance from the principal planes.

Prob. 6. A point 30 mm above xy line is the planview of two points P and Q. The elevation of P is 45 mm above the H. P. while that of the point Q is 35 mm below the H. P. Draw the projections of the points and state their position with reference to the principal planes and the quadrant in which they lie.